Molecular Moment Research Articles

Correlation between the glass transition temperatures and multipole moments for polymers

By carrying out density functional theory (DFT) calculations for 60 polyvinyls, polyethylenes and polymethacrylates repeating units at the B3LYP/6-31G(d) level, two quantum chemical descriptors, the molecular traceless quadrupole moment Θ, the molecular average hexadecapole moment Φ, are obtained and used to predict the glass transition temperature ( T g). A physically meaningful quantitative structure–property relationship (QSPR) model having correlation coefficient 0.953 for the training set and 0.952 for the test set is generated using multiple linear stepwise regression analysis. It suggests that the model obtained here can predict the T g values of polymers and provide theoretical guidance for polymeric molecular designs. Compared with the existing QSPR models, the proposed model with only two multipole moment descriptors is most simple. This paper encourages the further application of multipole moments to the QSPR studies for polymers.

Manipulation of slow molecular beams by static external fields

Deflection by magnetic or electric field gradients has long been used to analyze or to alter the translational trajectories of neutral gas-phase atoms or molecules. Recent work has developed sources of slow, cold molecular beams that offer means to enhance markedly the attainable deflections, which are inversely proportional to the translational kinetic energy. The sensitivity and resolution can thus be much increased, typically by factors of 10(2)-10(4). We illustrate ways to exploit this enhanced deflection capability, particularly when balancing electric and magnetic deflections. Chemical scope can be greatly extended by utilizing feeble but ubiquitous interactions, especially the induced electric dipole due to the molecular polarizability and magnetic moments resulting from molecular rotation or nuclear spins. We also examine the effect of non-Maxwellian velocity distributions produced by supersonic expansions or by quantum statistics (pertinent for ultracold beams). Generic plots are provided, employing dimensionless variables, to facilitate the design and interpretation of experiments with deflections amplified by low kinetic energy.

Novel Properties from Experimental Charge Densities: An Application to the Zwitterionic Neurotransmitter Taurine

The charge distribution of taurine (2-aminoethane-sulfonic acid) is revisited by using an orbital-based method that describes the density in a fixed molecular orbital basis with variable orbital occupation numbers. A new neutron data set is also employed to explore whether this improves the deconvolution of thermal motion and charge density. A range of molecular properties that are novel for experimentally determined charge densities are computed, including Weinhold population analysis, Mayer bond orders, and local kinetic energy densities, in addition to charge topological analysis and quantum theory of atoms-in-molecules (QTAIM) integrated properties. The ease with which a distributed multipole analysis can be performed on the fitted density matrix makes it straightforward to compute molecular moments, the lattice energy, and the electrostatic interaction energies of molecules removed from the crystal. Results are compared with high-level (QCISD) gas-phase calculations and band structure calculations employing density functional theory. Finally, the avenues available for extending the range of molecular properties that can be calculated from experimental charge densities still further using this approach are discussed.

Rigid Linkers for Bioactive Peptides

Rigid linkers of variable length were used to connect two high-affinity Nle4-D-Phe7-alpha-melanocyte stimulating hormone (NDP-alpha-MSH) or two low-affinity MSH(4) ligands. The linked peptides were synthesized by solid-phase methods. Control experiments indicate there is little or no effect of these linkers on NDP-alpha-MSH or MSH(4) binding to the human melanocortin 4 receptor (hMC4R). Tethering two high-affinity ligands gave no binding enhancement, while tethering two low-affinity ligands resulted in binding enhancement that decreased with increased linker length. Furthermore, for the low-affinity ligands, the enhancement of affinity is inversely proportional to the estimated molecular moments of inertia. These results are consistent with a model wherein binding is enhanced when the rate of ligand reattachment to the receptor is fast relative to the rate of ligand diffusion.

Laser-assisted ultracold lithium-hydride molecule formation: stimulated versus spontaneous emission

We investigate the feasibility of forming ultracold LiH from a mixture of the ultracold atomic gases, by using B1Π as an intermediate state in the photoassociation process. Using accurate molecular potential energy curves and dipole transition moments, we calculate and compare two possible schemes to populate vibrational levels of the ground electronic state, X1Σ+: (1) two-photon stimulated radiative association and (2) excitation to bound levels of the B1Π state, followed by spontaneous emission to the X1Σ+ state. With laser intensities and atomic densities that are easily attainable experimentally, we find that significant quantities of molecules can be formed in various v, J levels of the electronic ground state. We examine the spontaneous emission cascade which takes place from the upper vibrational levels on a time scale of milliseconds. We discuss the issue of back-stimulation for the two-photon process and ways to mitigate it. Because photon emission in the cascade process does not contribute to trap loss, a sizable population of molecules in v = 0 can be achieved.

Modeling the continuous melt transesterification of bisphenol-A and diphenyl carbonate in a continuous stirred tank reactor

A model of continuous melt transesterification of bisphenol-A and diphenyl carbonate in a continuous stirred tank reactor is developed using phase equilibria assumption and the method of molecular weight moments. The model equations can be simplified into a polynomial system that has 17 equations and 17 unknowns. Solution of the polynomial system gives out almost every aspects of the continuous transesterification process. Molecular weight and polydispersity index, end group ratio of hydroxyl to phenyl carbonate, contents of molecular species, and lost diphenyl carbonate fractions are studied in different operation parameters.

Possibility of 0-g-factor paramagnetic molecules for measurement of the electron’s electric dipole moment

The electric-field-dependent $g$ factor of the ground state of $^{2}\ensuremath{\Pi}_{1∕2}$ molecules is predicted cross zero at a specific value of electric field. Furthermore, $Edg∕dE$ evaluated at this field is expected to be small, allowing for extremely precise zeroing of the molecular magnetic moment. For the PbF molecule, we estimate this field to be $68\phantom{\rule{0.3em}{0ex}}\mathrm{kV}∕\mathrm{cm}$. If this prediction is correct, PbF could provide a uniquely sensitive probe of the electric dipole moment of the electron.

The vibrational structure of (E,E′)-1,4-diphenyl-1,3-butadiene: Linear dichroism FT-IR spectroscopy and quantum chemical calculations

The title compound (DPB) was investigated by FT-IR spectroscopy in liquid solutions and by FT-IR linear dichroism (LD) measurements on samples aligned in stretched polyethylene. The LD data provided experimental assignments of molecular transition moment directions and vibrational symmetries for more than 40 vibrational transitions. The observed IR wavenumbers, relative intensities, and polarization directions were generally well reproduced by the results of a harmonic analysis based on B3LYP/cc-pVTZ density functional theory (DFT). The combined experimental and theoretical results led to proposal of a nearly complete assignment of the IR active fundamentals of DPB, involving reassignment of a number of transitions. In addition, previously published Raman spectra of DPB were well predicted by the B3LYP/cc-pVTZ calculations.

Quantum-Admixture Model of High-Spin ↔ Low-Spin Transition for Ferrous Complex Molecules

A quantum-admixture model for the d(6) configuration ferrous complex molecules with the high-spin <--> low-spin transition has been established by using the unified crystal-field-coupling (UCFC) scheme. A general study has been made on the spin transition of octahedrally coordinated d(6) complexes, and a special application has been given to an Fe(II) compound Fe(II)(TRIM)(2)(PhCO(2))(ClO(4)). The results show the following: (i) The quantum picture of the spin transition of a d(6) system, such as Fe(II), is much more complex than a simple transition between the pure (5)T(2g) and (1)A(1g) states as usually understood. In practice, owing to spin-orbit coupling, spin is no longer a good quantum number and there is no longer a pure (5)T(2g) or (1)A(1g) state. Each of them splits into substates and each substate is a linear combination of various multiplets. The high-spin --> low-spin transition of an octahedrally coordinated d(6) ion is practically the crossover of the two lowest substates of (5)T(2g) at the critical point. (ii) At the spin-transition critical point the magnetic moment mu(eff) approximately 5.22 mu(B), which is obviously different from the simple average of the mu(eff) values of high-spin and low-spin states but near the saturation value. (iii) The calculation of the effective molecular magnetic moment mu(eff) for an octahedrally coordinated Fe(II) ion shows that the mu(eff)-T curve is in good agreement with Lemercier et al.'s experiment and both the low-spin value mu(eff) = 0.51 mu(B) and the high-spin value mu(eff) = 5.4 mu(B) are comparable with the experimental values 0.76 mu(B) and 5.4 mu(B), respectively. (iv) The T dependence of the crystal field parameter Dq in the spin-transition region is approximately linear.

MolecularCP-violating magnetic moment

A concept of CP-violating ($T$,$P$-odd) permanent molecular magnetic moments ${\ensuremath{\mu}}^{CP}$ is introduced. We relate the moments to the electric dipole moment of electron (eEDM) and estimate ${\ensuremath{\mu}}^{CP}$ for several diamagnetic polar molecules. The moments exhibit a steep, ${Z}^{5}$, scaling with the nuclear charge $Z$ of the heavier molecular constituent. A measurement of the CP-violating magnetization of a polarized sample of heavy molecules may improve the present limit on eEDM by several orders of magnitude.

A Clear-Cut Experimental Method to Discriminate between In-Plane and Out-of-Plane Molecular Transition Moments

A new method to orient organic molecules, based on their absorption as guest of the crystalline nanoporous delta phase of uniaxially stretched syndiotactic polystyrene films, is presented. This molecular alignment method not only allows high degrees of guest orientation to be attained but also orients the planar guest molecules with their smallest cross section nearly parallel to the stretching direction (B) rather than perpendicular (A, as usual for absorption in amorphous polymeric phases). As a consequence, in-plane and out-of-plane transition moment vectors maximize their absorption intensities for light polarization nearly perpendicular and parallel to the stretching direction, respectively. Hence, simple linear dichroism measurements by polarized spectra can allow an easy and clear-cut discrimination between in-plane and out-of-plane guest transition moment vectors.

Pressure Effects on the Structure and Phase Behavior of Phospholipid–Polypeptide Bilayers – A Synchrotron Small-Angle X-ray Scattering and 2H-NMR Spectroscopy Study on DPPC–Gramicidin Lipid Bilayers

Abstract The influence of gramicidin D (GD) incorporation on the structure and phase behavior of aqueous dispersions of fully hydrated phospholipid bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholin (DPPC) has been studied using small-angle X-ray scattering (SAXS) and 2H-NMR spectroscopy. The experiments covered a temperature range of 0–80 °C and a pressure range of 0.001–4 kbar. Pressure has been applied so as to be able to tune the lipid bilayer thickness at constant thermal energy and to select different gel phases, but also because high pressure is an important feature of certain natural membrane environments. SAXS was used to detect lipid bilayer topological changes upon incorporation of GD into the lipid bilayer. The data show that, depending on the GD concentration, the structure of the temperature- and pressure-dependent lipid phases is significantly altered by the insertion of the polypeptide, but also the lipid matrix has the ability to modulate the conformation of the inserted polypeptide. 2H-NMR spectroscopy yields further information on the conformation and dynamics of the lipid acyl-chains as a function of temperature, pressure and peptide concentration by measuring the molecular chain order parameters and spectral moments.

Solid-State Structures and Magnetic Properties of Halide-Bridged, Face-to-Face Bis-Nickel(II)-Macrocyclic Ligand Complexes: Ligand-Mediated Interchanges of Electronic Configuration

Dramatic differences are found between the ambient and 100 K X-ray structures of [L(2)Ni2Br2](ClO4)2 (L(2) = alpha,alpha'-bis{(5,7-dimethyl-1,4,8,11-tetraazacyclotetradeca-6-yl)-o-xylene), in which the bromide-bridged, bimetallic, macrocyclic ligand complexes of nickel(II) are held face-to-face and in which each bimetallic complex has a net triplet spin multiplicity. The ambient structure of this complex consists of very highly ordered, infinite chains of alternating R and S isomers in which the identical Ni(II) coordination spheres are near to the average expected for the high- and low-spin Ni(II) coordination sites, and there is appreciable stereochemical strain in the linkage of the macrocyclic ligands to the phenyl ring. In contrast, every other dinickel complex of the 100 K structure is displaced about 40 pm along the infinite chains to form tetrameric repeat units (pairs of dinickel complexes), in which each dinickel complex has well-defined high-spin and low-spin Ni(II) coordination sites; the high-spin sites are adjacent in the tetramers, and the stereochemical strain in the linkage to the phenyl spacer is relaxed. The molecular magnetic moments and structural contrasts are similar for the 100 K structure and the previously reported ambient structure of [L(2)Ni2Br3](ClO4) complex for which the molecular magnetic moments also correspond to a single triplet state per complex. The halide-bridged, monochloro- and monobromo dinickel complexes also have triplet spin multiplicity, and they crystallize with a coordinated perchlorate completing the axial coordination of the high-spin Ni(II) site, while the other Ni(II) site of these halide-bridged complexes has equatorial Ni-N bond lengths typical of low-spin Ni(II) coordination. The bridging halide is sandwiched between the face-to-face macrocyclic ligand Ni(II) moieties and slightly off the Ni-Ni axis in all of the complexes. The temperature dependence of the magnetic moments of the series of complexes indicates that their singlet-triplet energy gaps are small, with zero point energy differences that are generally less than 10(3) cm(-1). The very weak metal-metal electronic coupling, the triplet state spin multiplicity of each dinickel complex, and the averaged high-spin/low-spin coordination environments of the ambient structure implicate a vibronic mechanism for the electronic configurational exchange in the dibromo and tribromo complexes. The single molecular vibrational mode that correlates with the configurational exchange in these complexes includes the concerted motion of the bridging bromide between the Ni(II) centers. Activation of this vibrational mode is sufficient to effect the configurational exchange. These complexes present especially clear examples of the effects of the coupling of nuclear vibrational motions to the interchange of electronic configuration between two different centers.

A charge–charge flux–dipole flux decomposition of the dipole moment derivatives and infrared intensities of the AB 3 (A = N, P; B = H, F) molecules

The quantum theory of atoms in molecules (AIM) has been used to decompose dipole moment derivatives and fundamental infrared intensities of the AB 3 (A = N,P; B = H,F) molecules into charge–charge flux–dipole flux (CCFDF) contributions. Calculations were carried out at the MP2(FC)/6-311++G(3d,3p) level. Infrared intensities calculated from the AIM atomic charges and atomic dipoles are within 13.8 km mol −1 of the experimental values not considering the NH 3 and PH 3 stretching vibrations for which the experimental bands are severely overlapped. Group V atomic dipoles are very important in determining the molecular dipole moments of NF 3, PH 3 and PF 3 although the atomic charges account for almost all of the NH 3 molecular moment. Dipole fluxes on the Group V atom are important in determining the stretching band intensities of all molecules whereas they make small contributions to the bending mode intensities. Consideration of dipole flux contributions from the terminal atoms must also be made for accurately describing the intensities of all these molecules. As expected from a simple bond moment model, charge contributions dominate for most of the NH 3, NF 3, and PF 3 dipole moment derivatives and intensities. Charge flux and dipole flux contributions are very substantial for all the PH 3 vibrations, cancelling each other for the stretching modes and reinforcing one another for the bending modes.

Theoretical interpretation of electro-absorption spectra for intense optical transitions

The interaction of a light wave with a molecular crystal subjected at the same time to the influence of static electric field is analyzed. The coupling of the crystal to the radiation field is described in terms of classical electrodynamics, the molecular transition moments being represented by oscillating dipoles. The molecular parameters that enter the classical equations of motion (transition energy and oscillator strength), modified by the static electric field, are derived from the corresponding zero-field values using quantum-mechanical perturbation theory. Subsequently, the field-induced change of the absorption spectrum [electro-absorption (EA) signal] is calculated as the difference between the absorption spectra at nonzero and at zero modulating field. The approach is valid for any allowed transition, irrespective of its intensity. The results demonstrate that, while the absorption spectra of very intense transitions exhibit substantial peculiarities (such as orientational dispersion and polariton effects), the relationship between the absorption and electro-absorption spectra is always the same, regardless of the oscillator strength; specifically, the EA signal of a nondegenerate Frenkel exciton follows the first derivative of the corresponding absorption band. These conclusions are discussed in the context of recent literature on this subject.

Studies of Langmuir and Langmuir–Blodgett films of NLO-active amphiphilic 1,3-indanedione derivatives

We studied Langmuir and Langmuir–Blodgett (LB) films of several strong nonlinear optical (NLO) active amphiphilic derivatives of 1,3-indanedione-5,6-dicarboxylic acid. The surface pressure–area (π–A) isotherms at the air–water interface were investigated at different temperatures and pH values. Non-centrosymmetric Z-type LB films were deposited. UV–visible spectra indicated a uniform film transfer. The orientation of the molecular transition moments was calculated from the polarized UV–visible spectra. Packing within the LB films was observed by UV–visible absorption and fluorescence spectroscopy. The morphology of the LB films was examined by AFM and compared to ellipsometric measurements, and the nature of the films was studied by contact angle measurements. Significant second harmonic generation (SHG) by the LB films was observed. Alternate layer Y-type LB films of two different dyes were fabricated to achieve enhanced stability and SHG.

Calculation method of molecular weight averages in polymerization with chain-length-dependent termination

A systematic method for calculating the molecular weight distribution moments in free radical polymerization where termination rate depends on the size of the participating radicals, is presented. The central part of the method is the evaluation of the distribution of termination rates in the balance equations. From an adopted functional form of the termination rate constant, the moment equations are derived. For evaluating the moments of the termination rate distribution an approximate reconstruction of the radical chain length distribution using Laguerre polynomials is proposed. The calculation method can handle termination by disproportionation and combination simultaneously and allows easily to take into account diffusioncontrolled initiation, propagation and chain transfer reactions. The usefulness of the method is illustrated by simulating the bulk polymerization of methyl methacrylate and styrene. The calculated results of conversion, molecular weight averages (Mn,Mw,Mz and Mz+1) and polydispersity are in good agreement with the reported experimental data.

Structural connotations of bioactivity in a series of organophosphinates

AbstractPretreatment before exposure is one of the options for temporarily protecting persons liable to exposure to toxic organophosphorus compounds in agricultural or warfare situations. It is known that organophosphinates interact with neuronal cholinesterases, but that the latter may spontaneously reactivate in time. Before that reactivation, the enzyme is protected against comlexation with organophosphates. In this study, geometrically optimized unitary molecular indices, i.e., the molecular transforms, FTm, FTe, and FTc, indicating general, electronic, and charge properties, respectively, and the analogous normalized molecular moments, Mn, Me, and Mc, were calculated for a number of phosphinates. These indices were subsequently used in correlation trials with spontaneous reactivation percentages at specific elapsed times, as well as in clustering procedures, to evaluate the effect of structure variations on the reactivation percentages. The results of these studies are discussed, as is the effect of the octanol/water partition coefficient on the noted bioactivity. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Formation of ultracold polar molecules via Raman excitation

Alkali hydride molecules are very polar, exhibiting large ground-state dipole moments. As ultracold sources of alkali atoms, as well as hydrogen, have been created in the laboratory, we explore theoretically the feasibility of forming such molecules from a mixture of the ultracold atomic gases, employing a two-photon stimulated radiative association process -- Raman excitation. Using accurate molecular potential energy curves and dipole transition moments, we have calculated the rate coefficients for populating all the vibrational levels of the X $^1{\rm\Sigma}^ + $ state of LiH via the excited A $^1{\rm\Sigma}^ + $ state. We have found that significant molecule formation rates can be realized with laser intensities and atomic densities that are attainable experimentally. Because of the large X state dipole moment, rapid cascade occurs down the ladder of vibrational levels to v = 0. The calculated recoil momentum imparted to the molecule is small, and thus negligible trap loss results from the cascade process. This allows for the build-up of a large population of v = 0 trapped molecules.

Noncollinear magnetism in liquid oxygen: A first-principles molecular dynamics study

We perform first-principles molecular dynamics of liquid oxygen in which the magnetic structure evolves according to a generalized density-functional scheme allowing for noncollinear spin configurations. We investigate both structural correlations between the orientations of the molecular axes and magnetic correlations between the orientations of the molecular magnetic moments, demonstrating a clear relation between the local molecular configuration and the relative magnetic arrangement. The nuclear structure factor obtained from the simulation is found to agree well with the experimental one. The calculated magnetic structure factor shows antiferromagnetic correlations between molecules in the first shell, in accord with spin-polarized neutron scattering measurements. We observe the formation of dynamically coupled molecules, known as ${\mathrm{O}}_{4}$ units, in which the molecular moments are aligned in an antiferromagnetic fashion. An analysis based on the life time of such units, revealed that in most cases the ${\mathrm{O}}_{4}$ units occur as transient configurations during collisions. However, we also observed a small fraction of ${\mathrm{O}}_{4}$ units surviving for relatively long periods. To account for electronic excitations which are missed in our density-functional scheme, we complement our description with a mean field model for the thermal fluctuations of the magnetic structure. The combined scheme is found to improve the description of the magnetic neutron structure factor and allows us to study the dependence of the magnetic susceptibility on temperature.